We have a standard NUnit build step configured, with NUnit 3 selected, and the runner configured to use the runner in “PackagesNUnit.Console.3.0.1toolsnunit3-console.exe” and the extra nunit_use_project_file property configured as described in the post above.
Our total spec-running time is now ~8 minutes. Under NUnit 2 it was about 13 minutes.

What platform is your build environment using? How do you run your SpecFlow scenarios within it?

I have just spent a painful 3 days trying to migrate from NUnit 2.6.4 and SpecFlow 1.9. I had to give up with the NUnit migration due to problems with running the SpecFlows (which generate NUnit tests under the covers) on TeamCity.
I could not get the TeamCity.SpecFlow.Reporting PowerShell scripts to successfully run a feature. The NUnit 3.2 runner threw an exception every time I tried it. If I took the NUnit command that was being attempted and ran it in a command window it executed fine. The only difference that I can see between running NUnit within TeamCity and within a command window is that NUnit and TeamCity now cooperate behind the scenes when NUnit detects that it is running within TeamCity. Having failed with this approach, I then tried running the NUnit test fixtures produced by SpecFlow directly within TeamCity. This worked but it was extremely slow. My CI build times increased from 14 minutes to nearly 90! It appears to be so slow because when running like this every line of console output produced by the test fixtures is being added to the TeamCity build log (automatically by the NUnit/TeamCity cooperation). SpecFlow features and scenarios produce a lot of console output…